There were two for me:
John F. Kennedy...I was in readng class in middle school. I remember it like it was yesterday...where I was, how the nation reacted, how they grieved, how little John saluted his casket at the cemetary and all the controversy. I am just sorry that I will never know the truth in my lifetime since the official reports were ordered sealed for 100 years. I never believed for a minute though that is was the actions of just one man.
9/11...I was working as school nurse and office manager. I will never forget, particularly Flight 93 which crashed near Shanksville, PA. I am originally from PA and my brother-in-law is the Fire Chief of Shanksville so was one of the first responders to the scene. He and his son played a major part in helping with body recovery efforts. My b-i-l also helped with co-ordination with the FBI and higher powers to be that later arrived.
I have been to the site and have seen the photos my b-i-l and family took immediately after the crash...the large crater, which is no longer there. Just going to the site is very emotional. There is a wire fence with expressions of sympathy tied all over it. My b-i-l's fire jacket is hanging on the fence as well. The items are taken down before they become too tattered and worn from the elements and are stored for when the memorial is built. As fast as they are removed, new ones take their place. There are benches there for looking out over the scene and the benches have the names of the victims engraved on them. There are angels and flags as well.
My sister-in-law is a volunteer host at the site and explains what happened that day to the visitors. The son is a hired host at the site. You can't get very close as there are still artifacts that are probably buried and, of course, they don't want people trying to dig any of the things up or disturbing the area. I suggest you go there if you ever get to PA. I can never go without tearing up...it is very emotional.

There's a lot. I was in school when the space shuttle Challenger exploded. I was sitting in my house on a Naval base when the school shootings took place in Columbine. I was just getting in my truck when I heard about the first plane in New York, got home just in time, sad to say, to see the 2nd one hit the 2nd tower. I was at my doctor's appt. when the 3rd plane hit the the Pentagon,and the last plane crashed in PA. I was also at home when the space shuttle Columbus blew up. I pray I don't have to see anymore, or that my kids will.